Electronic devices that are used in high radiation environments, such as in outer space, are subject to many forms of radiation. The effects of ionizing radiation can accumulate over time, resulting in device degradation. Also, heavy ion strikes can lead ted catastrophic failure. When power devices are employed in such environments, the devices are typically more susceptible to these problems because of their large depletion volumes and large device areas.
Radiation hardened power MOSFETS, and other MOS gated devices designed for use in space or other high radiation ambients, have the conflicting design requirements of resisting damage caused by high doses of ionizing radiation on the one hand and of resisting damage caused by even single event high energy charged particles ("SEE") on the other. Thus, a thin gate oxide is desired to resist high radiation (megarad) environments, while a relatively thick gate oxide is desired to resist SEE effects.
More specifically, it is known that after exposure to a large total dose of ionizing radiation a positive charge will build up in the gate oxide to change the device threshold voltage. Further, there is an increase of interface traps at the silicon/gate oxide boundary. Both of these effects are reduced by using at thinner gate oxide, for example, one having a thickness of less than about 900 .ANG..
Devices used in a high radiation environment, such as in outer space, are also subject to damage or failure if struck by even a single high energy charged particle. Such charged particles pass into or through the silicon and generate a large number of electron-hole pairs in the depletion region of the device. Some of these charges collect on the gate oxide, resulting in a high potential across the gate oxide. Thus, a thicker gate oxide, for example, one thicker than about 1300 .ANG. is desired to resist SEE failure.
Because of these diverse requirements, different manufacturing processes are used for a "megarad" product designed for use in a high total radiation dose environment and an SEE product which is optimized for single particle effects.
In the known vertical conduction, multi-cellular MOSFET products, the charge collection at the oxide interface is in the drift region between cells. The device voltage is set in the charge in the inversion region. Thus, a design trade-off is necessary to set the gate oxide thickness for either a thin gate oxide for good total dose resistance or relatively thicker gate oxide for good SEE resistance.
It is also known that the P channel power MOSFET devices have demonstrated less susceptibility to SEE effects compared to N channel devices. G. H. Johnson, J. H. Hohl, R. D. Schrimpf and K. F. Galloway, "Simulating Single-Event Burnout in Vertical Power MOSFETs," IEEE Trans. Electron Devices, vol. 40 pp. 1001-1008, 1993. However, the threshold of P channel devices changes more rapidly with increasing total dose since both the accumulated oxide charge and interface traps cause the threshold to become more negative.
Furthermore, as noted above, optimizing the P-channel device to provide both SEE resistance and total radiation dose resistance requires significant trade offs. Typically, the threshold voltage shift is a monotonic function of the total radiation dose because the oxide charges and the interface traps make the threshold voltage more negative. As a result, the starting threshold voltage may need to be controlled to as near to -2V as possible. Further, the gate oxide should be kept as thin as possible to minimize positive charge buildup in the oxide. However, these requirements make the device more susceptible to single event gate rupture (SEGR) because of the thinner oxide. Also, the threshold voltage is typically a function of both the channel dopant density and the gate oxide thickness. When the channel doping level is too low, gain of the parasitic bipolar transistor increases, thereby increasing the risk of single event burnout. Therefore, total radiation dose protection capability favors incorporating thinner gate oxides and lower channel doping whereas the desire for SEE protection requires thicker gate oxides and higher channel doping.
It is thus further desirable to have a radiation hardened, P channel device that is optimized to maintain a predetermined threshold voltage at a high total irradiation dose while maintaining single event withstand capability.